Former prime minister Sir John Major has paid tribute to the Queen as an "absolute constant" in British public life.
Her Majesty will today surpass her great-great-grandmother Queen Victoria as Britain's longest reigning monarch after inheriting the throne in 1952.
Throughout the last 60 or so years, the Queen has been an absolute constant.
Whilst prime ministers have come and gone, celebrities have come and gone, life has changed, she and the monarchy have been an absolute constant in their lives and I think that is very reassuring.
Could overseeing the development of the Commonwealth be the Queen's greatest international success?
As Queen Elizabeth II becomes the country's longest reigning monarch, ITV News looks back at her remarkable rule.
Fashion and portrait photographer David Bailey said the Queen has the most recognisable face in the world and a "twinkle in her eye".